Cylindrical monocrystalline silicon ingots, grown through Czochralski (CZ) method, floating zone (FZ) method, etc., are served as basic materials for manufacturing semiconductor devices. Taking the CZ method for example, silica is heated to be melted in a crucible, and a rod-like seed crystal, about 10 mm in diameter, is then soaked in the melted silica. When the seed crystal is rotated and lifted gradually, the single crystal is grown with continued lattices constructed by silicon atoms from the melted polysilicon. If the environment is stable, the crystallization is carried out stably, and then eventually, a monocrystalline silicon ingot, cylindrical single crystal silicon, is formed. The ingot is then sliced, grinded, etched, cleaned and polished to form wafers.
Various kinds of defects may be formed during the growth of the monocrystalline silicon ingot, wafer and device fabrication due to imperfection of the monocrystalline, stress generated in the process or from the physical structure, or impurities coming from the environment, etc. For example, point defects in the form of vacancy and self interstitialcy, oxygen impurities from the quartz crucible, dislocations generated by stress or sheer force in semiconductor device fabrication, metal impurities introduced in the fabrication process and so on, may occur. Because the silicon atoms near the defects and interstitial silicon atoms own non-binding electron, and this causes interfaces or surfaces in the semiconductor devices dangling bonds. Number of carriers may be decreased by recombination or increased by generation here, depending on the bias voltage. Then, the electron mobility may be lowered to reduce the performance of the semiconductor devices. Similarly, metal impurities cause the semiconductor devices problems on the electrical characteristics, such as lower breakdown voltage, more leakage current, etc.
Another problem is hot carrier effect due to carrier injection. Carriers may get enough energy in high electric field, which relates to the small size of the semiconductor device, and then bombardments of the carriers may occur to allow a small portion of carriers entering the gate oxide. This may cause the semiconductor device low performance and unqualified reliability.
One of common solutions to solve these problems is hydrogen passivation treatment, in which the wafer formed with semiconductor devices is annealed in a hydrogen-rich environment to bind the dangling bonds with hydrogens. Through the hydrogen passivation treatment, the number of the dangling bonds as well as the undesirable effects on the operation of the semiconductor devices may be reduced. Another solution is deuterium conditioning, as disclosed in U.S. Pat. No. 5,872,387. After the fabrication of the semiconductor devices is finished, the semiconductor devices are then conditioned by deuterium gas. The deuterium atoms bind with Group III, IV or V elements in covalent bonds to form a stable structure. Thus, depassivation may be retarded, hot carriers may be reduced, leakage current may be decreased, and the performance and reliability of the semiconductor devices may be promoted. However, various kinds of dopants in the semiconductor devices may react with active hydrogen air or deuterium air at high temperature. This raises the difficulty to optimize the process parameters for passivation treatment or deuterium conditioning.
Yet, another choice is to generate oxygen precipitations in the wafers from residual interstitial oxygen atoms in a heating process. The oxygen precipitation may provide functions as a resource of intrinsic gettering for trapping metal impurities, avoiding dislocations from slipping, and promoting mechanical strength and performance of the semiconductor devices. However, if the oxygen precipitations occur in an active region of the semiconductor devices, the integrity of the gate oxide will be ruined, and this leads to leakage current which does not meet the requirement of modern semiconductor devices. Accordingly, the oxygen precipitations are necessarily formed outside the active region, such as bulk region, to avoid from hindering the operation of the semiconductor devices. To control the depth, density and size of the oxygen precipitations in a wafer is an important topic in this field.